Perceptions of Common Sense and Logical Thinking in Endodontics

Common sense generally implies the logical way to do things, a way that seems to those advocating it that any deviation from that path is either based on ignorance or some self-serving motivations. And there are times that that is exactly the case. Let’s see how perspectives on common sense in endodontic instrumentation play out. Today, rotary NiTi instrumentation either continuous or interrupted is accepted as the most advanced way to shape canals, that common sense dictates its dominant use, delivering results that are efficiently and predictably attained.  Rotary’s dominance underscores its common sense utility. If that were not the case, why would it be so dominant? This is a convenient form of circular reasoning that is useful in maintaining that dominance.

Facts clearly established in research and corroborated by clinical usage inform us that rotary NiTi is prone to instrument separation, that the rotary motion increases the exposure of the instruments to increased amounts of torsional stress and in curved canals cyclic fatigue the two factors most responsible for instrument separation. This was far less of a threat when instrumentation was confined to the short arcs of manual motion that were applied in pre-rotary times. Those supportive of rotary instrumentation may state that rotary instrumentation has become significantly less vulnerable to separation due to improved metallurgy, superior glide path preparation, more conservative preparations, single usage and the adoption of a reciprocating motion that lessens the impact of continuous rotation. And it is these evolving improvements that reinforce the claim by rotary NiTi advocates that it is the most common sense approach. 

Applied to common sense, instrumentation must not only be devoid of separation. It’s prime goal is to cleanse and shape the canals in three-dimensions with as little loss of tooth structure consistent with that goal. We know that greater tapered rotary preparations remove more tooth structure than necessary needlessly weakening the tooth’s resistance to vertical fracture. The more conservative use of rotary NiTi involves using lesser tapered instruments that sacrifice less dentin, are more flexible encountering less resistance and subject to a reduced amount of stress as they shave dentin away from the canal walls while negotiating to the apex. The concept of lesser-tapered rotary instrumentation makes sense as long as the space being cleansed is somewhat narrower than the instruments being employed. This is practically always the case in the mesio-distal plane, but many canals are far wider bucco-lingually. Under these circumstances, thin rotary NiTi instruments are so flexible that in those canal dimensions that are wider than the instrument being employed, a space exists that the instrument will deflect into when it first contacts the opposite canal wall. In short, instrumentation into the thinner bucco-lingual confines is compromised, a fact that has been demonstrated in several micro-ct scans recorded by Versiani and I have shown in previous posts. 

While advocates of rotary NiTi claim that they routinely apply their instruments against the buccal and lingual walls of elliptical canals to remove the tissue, the fact that they are so flexible nullifies this attempt and is generally not recommended by the manufacturers who state that staying centered is one of several keys in keeping the instruments intact. Rotary NiTi simply does not cut the mustard when thin isthmus-like pulpal configurations are present as demonstrated in Versiani’s micro-ct scans and to assume that merely leaning the highly flexible rotary NiTi instruments in the buccol-lingual plane will be effective is not supported in the research. These clinical realities based on bucco-lingual instrumentation undermine the common sense claims attributed to rotary NiTi. If used in a crown-down way with greater tapered preparations the technique sacrifices excessive amounts of dentin and will still not reach all the pulp tissue in elliptically shaped canals. If used conservatively they are likely to produce inadequate shaping and cleaning in the bucco-lingual plane of highly oval canals. 

When confronted with these micro-ct studies, the final bastion of defense for rotary NiTi advocates is the claim that the documented short-comings evidently do not effect the success rate which is claimed to be over 90%. This claim in no way removes the short-comings. The advocates are saying we can ignore these short- comings. If we really want to employ common sense, we would accentuate the fact that 90% success is not 100% success, that a 10% failure rate still leaves a lot of room for improvements, that these forms of improvement may come from cleansing and obturating the canals better, by reducing the incidence of dentinal defects produced in the canals via rotary instrumentation and by further reducing the incidence of instrument separation.

Common sense can pay dividends if we shift our focus from the rotary instruments employed after the creation of the glide path to the best tools available in the creation of the glide path and then examining just what we are seeking in the creation of that glide path. If we examine Versian’s micro-ct scans we see inadequate cleaning in the bucco-lingual plane, a result of the rotary NiTi instruments inability to penetrate these flatter more narrow aspects of the pulpal anatomy. If the tissue in the isthmus is not removed at this point in the cleansing process it will not be removed by subsequent instruments of increasing dimensions. So, the initial stages of glide path creation should be attempting to accomplish two goals, reaching the apex and penetrating as deeply bucco-lingually as possible in removing any pulp tissue present in that plane. From my years of practice, I find the most efficient tools to use at this stage of canal preparation are stainless steel twisted reamers that have dimensions as thin as 06/02. Even in these dimensions they have much more body than a comparably sized NiTi instrument giving it the ability to shave dentin away from the canal wall even where an opposite space exists without immediately deflecting into that space.

I attempt to negotiate to the apex with the first and likely thinnest stainless steel reamer (06/02) manually. At times of significant resistance, I will place the reamer in the 30o handpiece oscillating at 3000-4000 cycles per minute to ease my ability to reach the apex without incurring hand fatigue. Reaching the apex is my first goal and once at the apex, I may take as long as 20 seconds using a repetitive up and down motion with the oscillating handpiece to vigorously work against the buccal and lingual walls. Even with minor deflections, the high oscillation frequency effectively shaves dentin away from the canal walls. I apply an equal amount of time and force against all the canal walls in a technique I call internal routing that at best creates a shape that mirrors the original canal anatomy in larger form.

Proof of effectiveness of this approach is confirmed when I skip the 08/02 reamer and go directly to the 10/02 used in the oscillating handpiece and incur very little resistance to the apex and always far less than the original 06/02 reamer had encountered. This would not be possible if the 06/02 had not effectively shaved dentin away from all the canal walls. Once again, the reamers are applied vigorously against all the canal walls uniformly setting up the further enlargement of the canal in three-dimensions to a 15/02 and beyond all used in the 30o oscillating handpiece. The fact that stainless steel is stiffer than NiTi is irrelevant in these thin dimensions. Through a 20/02 the stainless steel reamers are more than flexible enough to traverse even highly curved anatomy without distortion. To further facilitate that ability, starting with the 15/02 a flat is incorporated along length giving it increased flexibility and less potential to cause distortions. The combination of the unrelieved stainless steel reamers through a 10/02 and relieved along length from 15/02 onwards used in the oscillating handpiece stay centered apically even when vigorously applied bucco-lingally following a pathway that is being increasingly defined by the previous instruments and creating a space that the subsequent instruments can faithfully follow. For me this is common sense in motion. 

This approach can be confined to the oscillating stainless steel reamers alone that routinely prepares canal shapes to a 35/02 or 40/02, but in three dimensions or it can be combined with a rotary system after the glide path has been shaped to a 15/02 or 20/02. Without any attempt to be unbiased, we developed a rotary NiTi system incorporating a helical flat that reduces engagement along the length of the rotary system shortening the time to reach the apex. Yet, I will say without equivocation, that if at any time employing the rotary NiTi reamers I encounter what I interpret as excessive resistance, I will intervene with the 30o oscillating system to widen the canals by the safest means possible and only after that continue with the rotary system for the final preparation of the canal. After final preparation with the rotary instruments, I will activate my final irrigant (Irritrol) a combination of EDTA and CHX with a 30/02 reamer in the oscillating handpiece to remove the smear layer and open up the dentinal tubules for better penetration of the sealer during obturation.

 There are many aspects of common sense in endodontics. I think the suggestions that I make are based on a strong foundation of common sense. For sure these techniques minimize deleterious effects and a statement that I have always been attracted to is to “first do no harm.”

Regards, Barry